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Designing a Q/V-Band Dual Circular Polarized Feed System

By RF Wireless Expert Team

Introduction : As satellite operators race to deploy next generation High Throughput Satellite (HTS) systems and mega constellations, they are moving gateway and feeder links out of the congested Ka-band. They have started using Q-band and V-band (37 GHz to 52 GHz) as these millimeter wave (mmWave) frequencies offer massive contiguous bandwidth.

To make Q/V-band SATCOM a reality, engineers must design highly advanced antenna feed systems. A standard requirement for these networks is a four port dual circular polarized feed system. This mechanical assembly allows an antenna to simultaneously transmit (Tx) and receive (Rx) signals using both Left Hand Circular Polarization (LHCP) and Right Hand Circular Polarization (RHCP) without the signals interfering with one another. This page describes three critical hardware components required to build next-gen mmWave feed system viz. Polarizer, OMT and Diplexer.

1. The Orthomode Transducer (OMT)

The Orthomode Transducer (OMT) is the heart of the feed network. Its job is to combine or separate orthogonal (i.e. perpendicular) electromagnetic fields. For a dual polarized system, it ensures that the horizontal and vertical signal components do not cross contaminate.

The Design Challenge:

Traditionally, high performance feed systems use a Bøifot OMT. This architecture offers excellent broadband isolation. However, its input ports are oriented orthogonally (at 90-degree angles to each other) and it typically uses a square antenna port. At Q/V-band frequencies, this requires bulky waveguide transitions to connect to the rest of the system, violating the need for a compact, lightweight assembly that fits behind a reflector dish.

The Solution : Turnstile OMT : To achieve a compact footprint, modern Q/V-band designs utilize a Turnstile OMT.

  • Symmetry: The turnstile configuration uses symmetrical junction that naturally divides the electromagnetic fields. This rotational symmetry provides exceptional amplitude and phase balance which suppresses unwanted signal modes and maintains polarization purity.
  • Coplanar Ports: Unlike the Bøifot, the turnstile design allows the output ports to be coplanar (i.e. on the same plane). This means the subsequent diplexers can be mounted side by side which simplifies mechanical integration to greater extent.
  • Circular Interface: A turnstile OMT naturally supports a circular waveguide interface, allowing it to connect directly to the circular polarizer without requirement of lossy transitions.

2. The Circular Polarizer

SATCOM networks rely on circular polarization as it prevents signal degradation caused by the Faraday effect in the ionosphere and misalignment between the satellite and the ground terminal. The polarizer’s job is to convert linear signals from the OMT into circular signals (and vice versa) with a very low axial ratio.

Various architectures have been evaluated such as dielectric polarizers, corrugated polarizers and quad-ridge circular polarizer.

For next-gen Q/V-band systems, quad-ridge circular waveguide polarizer is the optimal compromise. By carefully machining four stepped ridges inside a circular waveguide, engineers can achieve excellent broadband performance and highly stable, low axial ratio (e.g. 0.5 dB).

Moreover this design offer much shorter and more compact physical footprint compared to corrugated designs which makes it perfect for highly integrated feed assemblies.

3. The Frequency Diplexer

Once the OMT has separated the signal by polarization, the Diplexer must separate the signals by frequency. This is needed as the antenna is transmitting at V-band (e.g. 47–52 GHz) and receiving at Q-band (e.g. 37–42 GHz) simultaneously. The diplexer must provide extreme isolation (often >85 dB) so the massive Tx power doesn’t bleed into and destroy the highly sensitive Rx circuitry.

Instead of a standard T-junction, advanced Q/V-band diplexers use sidewall coupling. In this architecture, the main waveguide features a precisely dimensioned coupling slot on its side wall.

By meticulously controlling the physical length and dimensions of this coupling section, engineers can force the lower frequency receive signals to couple out into a separate port (acting as a high pass filter response). This eliminates the need for multiple, complex, independent bandpass filters, reducing the physical size of the diplexer while maintaining the immense port to port isolation required for simultaneous Tx/Rx operations.

Summary

Transitioning to the Q/V-band is not as simple as scaling down existing microwave components. Millimeter wave physics demands entirely optimized mechanical architectures. By combining Turnstile OMT, a compact Quad-Ridge Polarizer and Sidewall Coupled Diplexers, a four port feed system can be built which minimizes insertion loss, maximizes isolation and maintains polarization purity.

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